Connections of the facial and vestibular nerves: An anatomic study

Abstract

The facial and vestibulocochlear nerves emanate from the brain stem and then run parallel to each other within the internal auditory canal prior to their more peripheral distribution. Although anatomic connections between the facial and cochlear nerves have been described, reports outlining facial-vestibular anastomoses are few and may be found primarily in the non-English literature. The present study documents the existence of vestibulofacial neural connections as part of an anatomic dissection of 17 fresh human temporal bones.

Headnote

Abstract

The facial and vestibulocochlear nerves emanate from the brain stem and then run parallel to each other within the internal auditory canal prior to their more peripheral distribution. Although anatomic connections between the facial and cochlear nerves have been described, reports outlining facial-vestibular anastomoses are few and may be found primarily in the nonEnglish literature. The present study documents the existence of vestibulofacial neural connections as part of an anatomic dissection of 17 fresh human temporal bones.

Sommaire

Les nerfs faciaux et vestibulocochl6aires sortent ensemble du tronc: cerebral et se c6toient dans le conduit auditif interne avant de se separer en peripherie. Bien que les interconnexions entre ces nerfs ont ete decrites, les rapports les decrivant sont rares et presque inexistant en langue anglaise. Notre dissection de 17 os temporaux humains documente l'existence de ces connexions vestibulofaciales.

Key words: facial nerve, temporal bone, vestibulocochlear nerve, vestibulofacial nerve connections

he facial nerve exhibits a number of connections along its pathway through the temporal bone.' Within the internal auditory canal (IAC), it sends several small filaments to the acoustic nerve. At the level of the geniculate ganglion, communications are noted with the sphenopalatine and otic ganglia as well as with the sympathetic plexus through the greater petrosal nerve, lesser superficial petrosal nerve, and middle meningeal artery. At the opening of the stylomastoid foramen, there are delicate nerve fibres connecting to the vagus, auriculotemporal, and glossopharyngeal nerves. Peripheral branches of the facial nerve connect with the lesser occipital and cervicocutaneous nerves of the cervical plexus and with the trigeminal nerve at the cutaneous level of the face. In addition, the facial nerve forms a rich anatomic network within the substance of the parotid gland.2

Although the existence of vestibulofacial anastomosis within the IAC has been appreciated for a long time, a systematic anatomic investigation related to these potential connections requires further documentation. This report will attempt to study the anatomic relationship between the two nerves by demonstrating the existence of vestibulofacial neural connections within the IAC.

Material and Methods

Seventeen fresh temporal bones were examined. There was no history of middle or inner ear disease associated with any of the specimens. Other demographic data such as patient age, sex, and cause of death were not recorded.

After removing the middle fossa dura, the greater superficial petrosal nerve was identified and dissected. In all bones, the tegman tympani was drilled out in addition to the bone overlying the region of the geniculate ganglion. Thereafter, the roof of the IAC was exposed and drilled out leaving the underlying nerves within the canal covered by their fascial sheath. The fascia was then cut longitudinally under microscopic control exposing the facial, cochlear, and vestibular nerves. Neural connections emanating from the facial nerve were then documented and photographed.

Results

In 14 of 17 temporal bone dissections, there was clear microscopic evidence of connections between the facial and vestibular nerves. Histologic analysis indicated these connections to be neural rather than fibrous. The anastomotic fibres from the facial nerve connected to either the superior or vestibular nerves without any demonstrable pattern. For purposes of the present work, a few examples are shown:

Case 3 (Fig. 1). The cochlear nerve, which is the smallest and most anteroinferiorly located, and the facial and vestibular nerves were identified. A neural connection was demonstrated between the latter two nerves.

Case 7 (Fig. 2). The facial nerve was deflected posteriorly and superiorly so that the connection between the facial and vestibular nerves could be observed.

Case 14 (Fig. 3). The facial nerve was pushed forward and connections to the superior vestibular nerve were clearly seen.

Discussion

Our examination of 17 temporal bones clearly demonstrates the existence of various anatomic connections between the facial and vestibular nerves within the IAC. White and Sweet' in their book made reference to the possibility that the nervus intermedius may be the origin of anastomoses between cranial nerve VII and cranial nerve VIII in the distal portion of the IAC. This theory goes back to the detailed anatomic study of Bischof in 1865.1 It raises the question of whether these communications represent a constant pattern or whether these connections emerge in a rather haphazard way. Nevertheless, scattered reports have emerged outlining the anatomy and speculating on the functional or possibly physiologic aspects involving the connections between the facial and vestibular nerves.

Details of the anatomy of anastomoses have been forwarded by a number of authors. In 1951, Paturet4 proposed two patterns of connection having both consistent and inconsistent features. He described a constant pattern at a more posterior location connecting the nerve of Wrisberg to a branch of the vestibular. He also reported a constant pattern anteriorly connecting two or three fibres from the gems of the facial nerve to Scarpa's ganglion. In addition, he described connections between the facial and vestibular nerves in a more inconsistent manner medially near their origin at the brain stem. Other authors have made note of fibres running between the facial and vestibular nerves during their dissection of the IAC as part of their neurosurgical procedures.5-7

Figure I

Figure 2 1

Figure 3

Fisch6,7 speculated that the so-called vestibulofacial anastomosis may be seen and may be sectioned without any neurologic consequence.

There has been some speculation regarding the physiologic importance of these neural connections. In 1972, Wakasugis observed in some of his vertiginous patients that spontaneous nystagmus occurred as a minor side effect of facial nerve block for treatment of facial hemispasm. Such spontaneous nystagmus was noted on occasion when the local anaesthetic was injected close to the main trunk of the facial nerve. In these patients, both the vertigo and nystagmus disappeared within 60 minutes of the injection. To explain this phenomenon, he theorized that some of the local anaesthetic spread in retrograde fashion into these vestibulofacial anastomoses, thereby causing the transient vertigo and nystagmus. The presence of these connections was implicated by Fisch in 19729 to be the cause of a high rate of vestibular disturbance found in patients with Bell's palsy. He felt that the close proximity of the facial and vestibular nerves within the IAC including the presence of anastomotic connections between these structures contributed to the vestibular disturbance in facial paralysis cases. Although these are purely clinical observations, they do point to a functional type of relationship based on this anatomic connection between the two nerves.

In conclusion, anatomic evidence and clinical speculation clearly suggest a more than casual relationship between the facial and vestibular nerves. This work demonstrates the existence of neural anastomoses between the two nerves within the IACs of dissected temporal bones. The clinical significance of these findings requires further clarification.

References

References

References

1. Bischof EPE. Mikroskopische Analyse der Anastomosen der Kopfnerven. Munchen: JJ Lentner, 1865.

2. Conley J. Salivary glands and the facial nerve. New York: Grune & Stratton, 1975: 1-14.

References

3. White JC, Sweet WH. Pain and the neurosurgeon. Springfield, IL: Charles C. Thomas, 1969: 625-626.

4. Paturet G. Traite d'anatomie humaine. Vol. 1. Paris: Masson et Cie, 1951.

References

5. House W. Surgical exposure of the LA.C. Laryngoscope 1961; 71:1363-1385.

6. Fisch U. Transtemporal surgery of the internal auditory canal. Adv Otolaryngol 1970; 17:203-240.

7. Fisch U. Excision of Scarpa's ganglion. Arch Otolaryngol 1973. 97:147-149.

References

8. Wakasugi B. Facial nerve block in the treatment of facial spasm. Arch Otolaryngol 1972; 95-.356-359.

9. Fisch U. Total in& expos of the facial nerve. Arch Otolaryngol 1972; 95:33-341.